Plastic film for optical use, polarizing plate, and image display device

Abstract

The invention provides an optical plastic film, a polarizing plate, and an image display device. A plastic film for optical use, which has a first surface and a second surface positioned on the opposite side from the first surface, and which is characterized in that: in the flow direction and the width direction of the plastic film, the thickness of the second surface is greater than that of the first surface; the indentation hardness of the cross section on the first surface side, the indentation hardness of the cross section on the second surface side, and the indentation hardness of the cross section in the middle in the thickness direction satisfy a predetermined relationship.

Description

[0001] This application is a divisional application. The application number of its original application is 202080038704.1, the filing date is May 28, 2020, and the invention title is "Plastic Film for Optics, Polarizing Plate, and Image Display Device". Technical Field

[0002] The present invention relates to a plastic film for optics, a polarizing plate, and an image display device. Background Art

[0003] Various plastic films for optics are often used in optical components such as image display devices. For example, in an image display device having a polarizing plate on a display element, a plastic film (polarization element protective film) for protecting a polarization element constituting the polarizing plate is used.

[0004] The plastic film for an image display device typified by the polarization element protective film preferably has excellent mechanical strength. Therefore, as the plastic film for an image display device, a stretched plastic film is preferably used.

[0005] In addition, the stretched plastic film has excellent scratch resistance. Therefore, as disclosed in Patent Documents 1 and 2, stretched plastic films with increased elastic modulus have been proposed.

[0006] Prior Art Documents

[0007] Patent Documents

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 2019-8293 (Claim 4)

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 2018-538572 (Claim 7) Summary of the Invention

[0010] Problems to be Solved by the Invention

[0011] As disclosed in Patent Documents 1 and 2, a stretched plastic film with a high elastic modulus can improve scratch resistance when scratched with a hard object such as a pencil or a stylus. However, a stretched plastic film with a high elastic modulus usually produces scratches earlier than a stretched plastic film with a low elastic modulus when repeatedly rubbed with a soft object such as a cloth.

[0012] On the other hand, a stretched plastic film with a low elastic modulus has relatively good scratch resistance when repeatedly rubbed with a soft object such as a cloth, but immediately produces scratches when scratched with a hard object such as a pencil or a stylus.

[0013] As described above, improving the scratch resistance of the plastic film for both hard and soft objects is a trade-off relationship.

[0014] An object of the present invention is to provide a plastic film for optics, a polarizing plate, and an image display device, which can improve scratch resistance for both hard objects and soft objects.

[0015] Means for Solving the Problem

[0016] The present invention provides the following plastic film for optics, polarizing plate, and image display device.

[0017] [1] A plastic film for optics, which has a first surface and a second surface located on the side opposite to the first surface, and satisfies the following conditions.

[0018] <Condition 1>

[0019] Regarding the indentation hardness of the cross-section in the flow direction of the plastic film, the softer hardness among the indentation hardness of the cross-section on the first surface side and the indentation hardness of the cross-section on the second surface side is defined as MD1, and the indentation hardness of the cross-section at the exact middle in the thickness direction is defined as MD2. In addition, regarding the indentation hardness of the cross-section in the width direction of the plastic film, the softer hardness among the indentation hardness of the cross-section on the first surface side and the indentation hardness of the cross-section on the second surface side is defined as TD1, and the indentation hardness of the cross-section at the exact middle in the thickness direction is defined as TD2. On this premise, both MD2 / MD1 and TD2 / TD1 exceed 1.01 and are 1.30 or less.

[0020] [2] The plastic film for optics according to the above [1], which further satisfies the following Condition 2.

[0021] <Condition 2>

[0022] When the larger one of the product of MD1 and MD2 and the product of TD1 and TD2 is defined as X1 and the smaller one is defined as X2, X1 / X2 is 1.30 or less.

[0023] [3] The plastic film for optics according to the above [1] or [2], which further satisfies the following Condition 3.

[0024] <Condition 3>

[0025] A sample having a size of 50 mm in the flow direction × 50 mm in the width direction is cut out from the plastic film. The directions of the slow axes at five locations, namely, four locations advancing 10 mm from the four corners of the above sample toward the central part and the central part of the above sample, are measured. When the angles formed by either the flow direction or the width direction of the above sample and the directions of the slow axes at each measurement location are defined as D1, D2, D3, D4, and D5 respectively, the difference between the maximum value and the minimum value of D1 to D5 is 5.0 degrees or more.

[0026] [4] The plastic film for optics according to any one of [1] to [3] above further satisfies the following Condition 4.

[0027] <Condition 4>

[0028] Cut out a sample with a size of 50 mm in the flow direction × 50 mm in the width direction from the plastic film. Measure the in-plane retardation at four positions advancing 10 mm from the four corners of the above sample to the central part and at the central part of the above sample, a total of five positions. When the in-plane retardations at the above five positions are defined as Re1, Re2, Re3, Re4, and Re5 respectively, the average of Re1 to Re5 is 500 nm or less.

[0029] [5] The plastic film for optics according to any one of [1] to [4] above further satisfies the following Condition 5.

[0030] <Condition 5>

[0031] Cut out a sample with a size of 50 mm in the flow direction × 50 mm in the width direction from the plastic film. Measure the retardation in the thickness direction at four positions advancing 10 mm from the four corners of the above sample to the central part and at the central part of the above sample, a total of five positions. When the retardations in the thickness direction at the above five positions are defined as Rth1, Rth2, Rth3, Rth4, and Rth5 respectively, the average of Rth1 to Rth5 is 2000 nm or more.

[0032] [6] A polarizing plate having: a polarizing element; a transparent protective plate A disposed on one side of the above polarizing element; and a transparent protective plate B disposed on the other side of the above polarizing element, wherein at least one of the above transparent protective plate A and the above transparent protective plate B is the plastic film for optics according to any one of [1] to [5] above.

[0033] [7] An image display device having: a display element; and a plastic film disposed on the light-emitting surface side of the above display element, wherein the above plastic film is the plastic film for optics according to any one of [1] to [5] above.

[0034] [8] The image display device according to [7] above, which has a polarizing element between the above display element and the above plastic film.

[0035] Effects of the Invention

[0036] The plastic film for optics, polarizing plate, and image display device of the present invention can improve scratch resistance for both hard objects and soft objects. Description of the Drawings

[0037] Figure 1It is a perspective view of a sample for measuring the cross-sectional hardness of a plastic film.

[0038] Figure 2 It is a cross-sectional view showing the measurement positions of the cross-sectional hardness on the first surface side and the cross-sectional hardness on the second surface side under Conditions 1 and 2.

[0039] Figure 3 It is a top view showing the measurement positions of five parts under Conditions 3 to 6.

[0040] Figure 4 It is a cross-sectional view showing an embodiment of the image display device of the present invention.

[0041] Figure 5 It is a cross-sectional view showing another embodiment of the image display device of the present invention.

[0042] Figure 6 It is a diagram schematically showing the situation of a continuous folding test.

[0043] Symbol Explanation

[0044] 10: Plastic film for optics

[0045] 20: Display element

[0046] 30: Polarizer

[0047] 31: Polarizing element

[0048] 32: Transparent protective plate A

[0049] 33: Transparent protective plate B

[0050] 50: Housing

[0051] 100: Image display device

[0052] S: Cut sample of plastic film

[0053] R: Embedding resin

[0054] d: Thickness direction of the cut sample of the plastic film Detailed Embodiment

[0055] Hereinafter, embodiments of the present invention will be described.

[0056] [Plastic film for optics]

[0057] The plastic film for optics of the present invention has a first surface and a second surface located on the side opposite to the first surface, and satisfies the following conditions.

[0058] <Condition 1>

[0059] Regarding the indentation hardness of the cross-section in the flow direction of the plastic film, the softer hardness among the indentation hardness of the cross-section on the first surface side and the indentation hardness of the cross-section on the second surface side is defined as MD1, and the indentation hardness of the cross-section at the exact middle in the thickness direction is defined as MD2. Additionally, regarding the indentation hardness of the cross-section in the width direction of the plastic film, the softer hardness among the indentation hardness of the cross-section on the first surface side and the indentation hardness of the cross-section on the second surface side is defined as TD1, and the indentation hardness of the cross-section at the exact middle in the thickness direction is defined as TD2. On this premise, both MD2 / MD1 and TD2 / TD1 exceed 1.01 and are 1.30 or less.

[0060] <Measurement Regarding Condition 1 and Condition 2>

[0061] Condition 1 and Condition 2 stipulate the indentation hardness of the cross-section in the flow direction of the plastic film and the indentation hardness of the cross-section in the width direction of the plastic film.

[0062] Regarding Condition 1 and Condition 2, in order to measure the cross-section hardness of the plastic film, first, it is necessary to produce a sample for measurement. This sample is produced, for example, through the following processes (A1) to (A2).

[0063] (A1) Two cut samples are produced by cutting an optical plastic film into a size of 2 mm in the flow direction × 10 mm in the width direction. After that, two embedded samples are produced by embedding the cut sample S as Figure 1 shown with a resin R. The resin for embedding is preferably an epoxy resin.

[0064] The embedded sample can be obtained, for example, as follows: After arranging the cut sample in a silicon embedding plate (silicon capsule), the resin for embedding is poured. Furthermore, after curing the resin for embedding (in the case of the epoxy resin manufactured by Struers Company exemplified below, it is preferably left at room temperature for 12 hours for curing), the cut sample and the resin for embedding that embeds the cut sample are taken out from the silicon embedding plate (silicon capsule), and thus the embedded sample can be obtained. The silicon embedding plate (silicon capsule) can be, for example, the silicon embedding plate (silicon capsule) manufactured by DOSAKA EM Company. The epoxy resin for embedding can use, for example, a substance obtained by mixing the product name “EpoFix” manufactured by Struers Company and the product name “Curing Agent for EpoFix” manufactured by the same company at a ratio of 10:1.2. It should be noted that the two cut samples are cut from a close area (within a 50 mm × 50 mm area). Additionally, the sizes of the two cut samples are 2 mm ± 0.2 mm in the flow direction × 10 mm ± 1 mm in the width direction.

[0065] (A2) A sample embedded in a block is cut perpendicularly to the flow direction using a diamond knife to produce a sample for indentation hardness measurement of the cross-section in the flow direction, with the cross-section in the flow direction exposed. Another sample embedded in a block is cut perpendicularly to the width direction using a diamond knife to produce a sample for indentation hardness measurement of the cross-section in the width direction, with the cross-section in the width direction exposed. The embedded sample is preferably cut through the center of the cut sample.

[0066] As a device for cutting the embedded sample, for example, a product named "Ultramicrotome EMUC7" manufactured by Leica Microsystems can be cited.

[0067] Using the sample for indentation hardness measurement of the cross-section in the flow direction produced as described above, measure the indentation hardness of the cross-section on the first surface side, the indentation hardness of the cross-section on the second surface side, and the indentation hardness of the cross-section at the midpoint in the thickness direction, and calculate MD1 and MD2.

[0068] Similarly, using the sample for indentation hardness measurement of the cross-section in the width direction produced as described above, measure the indentation hardness of the cross-section on the first surface side, the indentation hardness of the cross-section on the second surface side, and the indentation hardness of the cross-section at the midpoint in the thickness direction, and calculate TD1 and TD2.

[0069] It should be noted that in this specification, the indentation hardness of the cross-section on the first surface side in the flow direction and width direction, the indentation hardness of the cross-section on the second surface side, and the indentation hardness of the cross-section at the midpoint in the thickness direction refer to the average of 5 measurement values.

[0070] Regarding the indentation hardness of the cross-section, a Berckovich indenter (material: diamond triangular pyramid) is vertically pressed into the cross-section of the above sample for measurement. Regarding the indentation hardness of the cross-section on the first surface side and the indentation hardness of the cross-section on the second surface side, as Figure 2 shown, the measurement is performed at a position 2.0 μm inside from the first surface and the second surface ([[]] Figure 2 the positions (i) and (iii) correspond to the measurement parts). It should be noted that [[[]] Figure 2 corresponds to [[[]] Figure 1 the xz cross-sectional view. In addition, [[[]] Figure 2 "d" refers to the thickness direction of the cut sample of the plastic film. In addition, [[[]] Figure 2 "(ii)" refers to the midpoint position in the thickness direction of the cut sample of the plastic film.

[0071] The indentation hardness is preferably measured under the following conditions.

[0072] <Measurement Conditions>

[0073] · Indenter used: Berckovich indenter (Model: TI-0039, manufactured by HYSITRON Inc.)

[0074] · Indentation condition: Displacement control mode

[0075] · Maximum indentation depth: 200 nm

[0076] · Loading time: 20 seconds (Speed: 10 nm / second)

[0077] · Holding time: Hold at the maximum indentation depth for 5 seconds

[0078] · Unloading time: 20 seconds (Speed: 10 nm / second)

[0079] The indentation hardness can be calculated as follows.

[0080] First, the indentation depth h (nm) corresponding to the indentation load F (N) is continuously measured to produce a load-displacement curve. Analyze the produced load-displacement curve. As the maximum indentation load F max (N) divided by the projected area A p (mm 2 ) of the indenter in contact with the plastic film, the value obtained can be used to calculate the indentation hardness H IT (as shown in the following formula (1)).

[0081] H IT =F max / A p …(1)

[0082] Here, A p is the contact projected area obtained by correcting the tip curvature of the indenter using the standard method of the device.

[0083] Before measuring the indentation hardness, it is preferable to perform standard adjustment.

[0084] The standard adjustment can be performed as follows, for example: Perform an indentation test using a standard specimen with known indentation hardness and complex elastic modulus, and confirm that the indentation hardness and complex elastic modulus obtained from the test results are within the reference values.

[0085] It is preferable to perform standard adjustment each time the sample is changed. However, if the samples are the same, from the perspective of work efficiency, it is preferable to continuously perform multiple measurements of the indentation hardness. That is, it is preferable to perform the measurement as shown in the following (1). It should be noted that in the following (1), the measurement of the indentation hardness of the cross-section in the flow direction and the measurement of the indentation hardness of the cross-section in the width direction can be interchanged.

[0086] (1) After implementing the above standard adjustments, for the samples used to measure the indentation hardness of the cross-section in the flow direction, measure the indentation hardness of the cross-section on the first surface side, the indentation hardness of the cross-section on the second surface side, and the indentation hardness of the cross-section at the exact middle in the thickness direction five times each, and calculate MD1 and MD2. Implement the above standard adjustments again. For the samples used to measure the indentation hardness of the cross-section in the width direction, measure the indentation hardness of the cross-section on the first surface side, the indentation hardness of the cross-section on the second surface side, and the indentation hardness of the cross-section at the exact middle in the thickness direction five times each, and calculate TD1 and TD2.

[0087] In addition, when continuously measuring the indentation hardness for a long time, it is preferable to implement the standard adjustment at least before 12 hours have passed. For example, even if the standard adjustment is not implemented every time the sample is changed, it is preferable to implement the standard adjustment at least before 12 hours have passed.

[0088] In this specification, for various measurement atmospheres such as Condition 1, Condition 2, and Conditions 3 to 6 described later, unless otherwise specified, the temperature is 23°C ± 5°C and the humidity is 40% to 65% RH. In addition, before the measurement, expose the sample to the above atmosphere for more than 30 minutes.

[0089] <Flow direction and width direction>

[0090] Optical plastic films, for example, have a sheet form and a roll form.

[0091] In the roll form, the flow direction of the roll and the width direction of the roll are easy to identify.

[0092] On the other hand, in the sheet form, when the flow direction and the width direction can be easily identified as in the case of a uniaxially stretched film, the flow direction and the width direction can be determined based on this identification (in the case of a uniaxially stretched film, usually the direction of the slow axis is the width direction).

[0093] When it is difficult to identify the flow direction and the width direction of the sheet, the flow direction and the width direction can be determined as follows in (1) and (2) below.

[0094] (1) When the sheet is rectangular or square, the flow direction or the width direction can be determined using the four sides that make up the rectangle or square. The sheet-like film is made by punching the roll-like film. And, in order to improve the yield of the sheet, it is necessary to punch the sheet along the flow direction and the width direction of the roll. Thus, when the sheet is rectangular or square, it can be said that the directions of the four sides that make up the rectangle or square are consistent with the flow direction or the width direction, which is common technical knowledge.

[0095] (2) When the sheet has a shape other than rectangular or square (ellipse, triangle, polygon other than triangle, etc.), draw the largest rectangle or square that does not protrude from the shape, and determine the flow direction or width direction in the same manner as in the above (1) based on the drawn rectangle or square.

[0096] It should be noted that in the determination of the above (1) and (2), it is impossible to distinguish which of the two directions is the flow direction and which is the width direction. However, since Conditions 1 to 6 in this specification are parameters that hold even if the flow direction and width direction are reversed as long as the two directions as the flow direction and width direction can be determined, the methods in the above (1) and (2) can be used to determine the flow direction and width direction.

[0097] <Condition 1>

[0098] Condition 1 stipulates that both MD2 / MD1 and TD2 / TD1 exceed 1.01 and are 1.30 or less.

[0099] Both MD2 / MD1 and TD2 / TD1 exceeding 1.01 mean that the indentation hardness of the cross-section of the plastic film is greater inside than on the surface side. When the surface hardness of two different objects is of the same degree, the object with a harder interior can improve scratch resistance when rubbing the surface of the object with a hard object. Therefore, by making both MD2 / MD1 and TD2 / TD1 exceed 1.01, scratch resistance against hard objects can be improved regardless of the rubbing direction.

[0100] On the other hand, both MD2 / MD1 and TD2 / TD1 being 1.30 or less means that the indentation hardness of the cross-section of the plastic film is not too large inside compared to the surface side. When rubbing the surface of the plastic film with a soft object, when the interior of the plastic film is soft, the stress during rubbing can be easily released. Therefore, by making both MD2 / MD1 and TD2 / TD1 1.30 or less, scratch resistance against soft objects can be improved regardless of the rubbing direction.

[0101] Under Condition 1, both MD2 / MD1 and TD2 / TD1 are preferably more than 1.01 and 1.20 or less, more preferably 1.02 or more and 1.15 or less, and further preferably 1.02 or more and 1.10 or less.

[0102] The absolute values of MD1, MD2, TD1, and TD2 are not particularly limited as long as they are in a range that gives appropriate mechanical strength, and are usually 150 MPa to 350 MPa, preferably 170 MPa to 300 MPa, more preferably 200 MPa to 270 MPa, and further preferably 220 MPa to 250 MPa.

[0103] One embodiment of the plastic film for optics of the present invention preferably further satisfies the following Condition 2.

[0104] <Condition 2>

[0105] When defining the larger one as X1 and the smaller one as X2 among the product of MD1 and MD2 and the product of TD1 and TD2, X1 / X2 is 1.30 or less.

[0106] A small X1 / X2 means a small anisotropy in hardness in the flow direction and the width direction. Therefore, by making X1 / X2 1.30 or less, it is possible to suppress the generation of scratches in a specific direction when an object (such as a pen tip) touches the plastic film (hereinafter, this property may sometimes be referred to as "scratch resistance"). In addition, by making X1 / X2 1.30 or less, it is possible to easily suppress the remaining bending marks or breakage after the bending test.

[0107] X1 / X2 is more preferably 1.25 or less, and further preferably 1.20 or less. It should be noted that the lower limit of X1 / X2 is about 1.03, preferably 1.05 or more, and more preferably 1.10 or more.

[0108] One embodiment of the plastic film for optics of the present invention preferably further satisfies the following Condition 3.

[0109] <Condition 3>

[0110] Cut out a sample of 50 mm in the flow direction × 50 mm in the width direction from the plastic film. Measure the directions of the slow axes at a total of 5 locations, namely 4 locations advancing 10 mm from the four corners to the central part of the above sample and the central part of the above sample. When defining the angles formed by either the flow direction or the width direction of the above sample and the directions of the slow axes at each measurement location as D1, D2, D3, D4, and D5 respectively, the difference between the maximum value and the minimum value of D1 to D5 is 5.0 degrees or more.

[0111] Condition 3 stipulates that the difference between the maximum value and the minimum value of D1 to D5 is 5.0 degrees or more. By making this difference 5.0 degrees or more, it is possible to suppress at least the occurrence of blackout in the area of the sample when observed with polarized sunglasses.

[0112] The existing plastic film for optics is designed such that the direction of the slow axis does not shift, but the plastic film that satisfies Condition 3 intentionally shifts the direction of the slow axis, which is different from the structure of the existing optical film. In addition, it can be said that the plastic film that satisfies Condition 3 is also characterized in that it focuses on the deviation of the slow axis in a relatively small area of 50 mm in length × 50 mm in width.

[0113] In addition, from the aspect of being able to improve the bend resistance of the plastic film, it is preferable to satisfy Condition 3.

[0114] On the other hand, for a general-oriented film with the slow axes aligned, the film breaks after a bending test or shows strong bending marks remaining. Specifically, a general uniaxially stretched film breaks when subjected to a bending test along the slow axis, and shows strong bending marks remaining when subjected to a bending test along the direction orthogonal to the slow axis. In addition, a general biaxially stretched film shows strong bending marks remaining when subjected to a bending test along the direction orthogonal to the slow axis.

[0115] The plastic film satisfying Condition 3 is preferred in that it can suppress the remaining bending marks or breakage after the bending test regardless of the bending direction.

[0116] The difference between the maximum value and the minimum value of D1 to D5 is preferably 6.0 degrees or more, more preferably 8.0 degrees or more, and further preferably 10.0 degrees or more.

[0117] It should be noted that if the difference between the maximum value and the minimum value of D1 to D5 is too large, there is a tendency for the orientation of the plastic film to decrease and the mechanical strength to decrease. Therefore, the difference is preferably 20.0 degrees or less, more preferably 17.0 degrees or less, and further preferably 15.0 degrees or less.

[0118] For the plastic film for optics according to one embodiment of the present invention, D1 to D5 are respectively preferably 5 degrees to 30 degrees or 60 degrees to 85 degrees, more preferably 7 degrees to 25 degrees or 65 degrees to 83 degrees, and further preferably 10 degrees to 23 degrees or 67 degrees to 80 degrees.

[0119] By making D1 to D5 be 5 degrees or more or 85 degrees or less respectively, blackout can be easily suppressed when observed with polarized sunglasses. In addition, by making D1 to D5 be 30 degrees or less or 60 degrees or more respectively, a decrease in mechanical strength due to a decrease in the orientation of the plastic film can be easily suppressed.

[0120] One embodiment of the plastic film for optics of the present invention preferably further satisfies the following Condition 4.

[0121] <Condition 4>

[0122] A sample with a size of 50 mm in the flow direction × 50 mm in the width direction is cut out from the plastic film. The in-plane retardations at 4 positions advancing 10 mm from the four corners to the central part of the above sample and the central part of the above sample are measured. When the in-plane retardations at the above 5 positions are defined as Re1, Re2, Re3, Re4, and Re5 respectively, the average of Re1 to Re5 is 600 nm or less.

[0123] Condition 4 stipulates that the average of Re1 to Re5 is 600 nm or less. By making the average of Re1 to Re5 600 nm or less, when observing with the naked eye, it is at least possible to easily suppress the spots (rainbow spots) of the rainbow pattern within the area of the sample.

[0124] The average of Re1 to Re5 is more preferably 300 nm or less, further preferably 250 nm or less, and even more preferably 200 nm or less. The lower limit of the average of Re1 to Re5 is not particularly limited, and is usually about 50 nm, preferably 100 nm or more.

[0125] Re1 to Re5 are each preferably 600 nm or less, more preferably 300 nm or less, further preferably 250 nm or less, and even more preferably 200 nm or less.

[0126] The difference between the maximum value and the minimum value of Re1 to Re5 is preferably 200 nm or less, more preferably 150 nm or less, and further preferably 100 nm or less.

[0127] The plastic film for optics according to one embodiment of the present invention preferably satisfies the following Condition 5.

[0128] <Condition 5>

[0129] A sample having a size of 50 mm in the flow direction × 50 mm in the width direction is cut out from the plastic film. The phase difference in the thickness direction at four positions advancing 10 mm from the four corners of the sample to the central part and the central part of the sample, a total of five positions, is measured. When the phase differences in the thickness direction at the above five positions are defined as Rth1, Rth2, Rth3, Rth4, and Rth5 respectively, the average of Rth1 to Rth5 is 2000 nm or more.

[0130] By satisfying Condition 5, it is possible to make the stretching degree of the plastic film for optics close to uniform biaxiality, and it is possible to improve the mechanical strength of the plastic film for optics. In addition, by satisfying Condition 5, it is possible to easily suppress blackout when observing obliquely through polarized sunglasses.

[0131] The average of Rth1 to Rth5 is more preferably 3000 nm or more, and more preferably 4000 nm or more. The upper limit of the average of Rth1 to Rth5 is about 10000 nm, preferably 8000 nm or less, and more preferably 7000 nm or less.

[0132] In addition, Rth1 to Rth5 are each preferably 2000 nm to 10000 nm, more preferably 3000 nm to 8000 nm, and further preferably 4000 nm to 7000 nm.

[0133] The difference between the maximum values and the minimum values of Rth1 to Rth5 is preferably 200 nm or less, more preferably 150 nm or less, and further preferably 100 nm or less.

[0134] The plastic film for optics according to one embodiment of the present invention preferably further satisfies the following condition 6.

[0135] <Condition 6>

[0136] The average of Re1 / Rth1, Re2 / Rth2, Re3 / Rth3, Re4 / Rth4, and Re5 / Rth5 is 0.10 or less.

[0137] A small ratio (Re / Rth) of the in-plane retardation (Re) to the retardation in the thickness direction (Rth) means that the stretching degree of the plastic film for optics is close to uniform biaxiality. Therefore, by making this ratio 0.10 or less, the mechanical strength of the plastic film for optics can be improved. This ratio is more preferably 0.07 or less, and further preferably 0.05 or less. The lower limit of this ratio is about 0.01.

[0138] Re1 / Rth1, Re2 / Rth2, Re3 / Rth3, Re4 / Rth4, and Re5 / Rth5 are each preferably 0.10 or less, more preferably 0.07 or less, and further preferably 0.05 or less. The lower limit of these ratios is about 0.01.

[0139] <Measurement of Conditions 3 to 6>

[0140] A sample with a size of 50 mm in length × 50 mm in width used under Conditions 3 to 6 is cut out from an arbitrary position of the plastic film. The measurement points at five positions under Conditions 3 to 6 are a total of five positions, namely one position at the central part and four positions that advance 10 mm from the four corners of the sample towards the central part ( Figure 3 the five black circular positions).

[0141] Regarding the in-plane retardation (Re) of Condition 4 and the retardation in the thickness direction (Rth) of Condition 5, they are represented by the refractive index nx in the slow axis direction, which is the direction with the maximum refractive index at each measurement part, the refractive index ny in the fast axis direction, which is the direction orthogonal to the above slow axis direction at each measurement part, the refractive index nz in the thickness direction of the plastic film, and the thickness T [nm] of the plastic film, by the following formulas (1) and (2). It should be noted that in this specification, the in-plane retardation (Re) and the retardation in the thickness direction (Rth) refer to the values at a wavelength of 550 nm.

[0142] In-plane retardation (Re) = (nx - ny) × T [nm] (1)

[0143] In-plane retardation (Rth) in the thickness direction = ((nx + ny) / 2 - nz) × T [nm] (2)

[0144] The direction of the slow axis, the in-plane retardation (Re), and the in-plane retardation in the thickness direction (Rth) can be measured, for example, using the product names "RETS-100" manufactured by Otsuka Electronics Co., Ltd., "KOBRA-WR" and "PAM-UHR100" manufactured by Oji Scientific Instruments Co., Ltd.

[0145] When measuring the in-plane retardation (Re), etc. using the product name "RETS-100" manufactured by Otsuka Electronics Co., Ltd., it is preferable to prepare for the measurement according to the following steps (A1) to (A4).

[0146] (A1) First, in order to stabilize the light source of RETS-100, turn on the light source and leave it for 60 minutes or more. After that, select the rotating analyzer method and select the θ mode (the mode for measuring the angular retardation and calculating Rth). Since this θ mode is selected, the stage is a tilting rotating stage.

[0147] (A2) Next, input the following measurement conditions into RETS-100.

[0148] (Measurement conditions)

[0149] · Retardation measurement range: Rotating analyzer method

[0150] · Measurement spot diameter:

[0151] · Tilt angle range: 0°

[0152] · Measurement wavelength range: 400 nm to 800 nm

[0153] · Average refractive index of the plastic film (for example, in the case of a PET film, N = 1.617)

[0154] · Thickness: Thickness measured separately using SEM or an optical microscope

[0155] (A3) Next, without setting the sample in the device, obtain background data. The device is a closed system, and this operation is performed each time the light source is turned on.

[0156] (A4) After that, set the sample on the stage in the device and perform the measurement.

[0157] Under Condition 3, as long as all of D1 to D5 use the same direction as the reference direction of the angle formed with the slow axis direction, either the flow direction or the width direction can be used as the reference.

[0158] When it is possible to cut out a plurality of samples each having a size of 50 mm in length and 50 mm in width from a sheet-like plastic film, the proportion of the samples satisfying the specified conditions among the plurality of samples is preferably 50% or more, more preferably 70% or more, further preferably 90% or more, and even more preferably 100% or more.

[0159] In addition, when it is possible to cut out a plurality of samples each having a size of 50 mm in length and 50 mm in width from a roll-shaped plastic film, it is preferable that most of the samples cut out from a specific position in the width direction of the roll satisfy the specified conditions in the flow direction of the roll. By satisfying this configuration, if the plastic film at a specific position in the width direction of the roll is picked up, a plastic film that exhibits a specified effect can be produced.

[0160] <Plastic film>

[0161] Examples of the laminated structure of the plastic film include a single-layer structure and a multilayer structure. Among them, a single-layer structure is preferable.

[0162] As described later, in order to suppress iridescence while improving mechanical strength, the plastic film is preferably a stretched plastic film with a small in-plane retardation. And, in order to reduce the in-plane retardation of the stretched plastic film, it is important to perform fine stretching control such that the stretching in the longitudinal and transverse directions is equally close. Regarding the fine stretching control, it is difficult to perform this control in a multilayer structure due to differences in physical properties of each layer, etc., but it is easy to perform this control in a single-layer structure, and thus it is preferable from this aspect.

[0163] Examples of the resin component constituting the plastic film include polyester, triacetyl cellulose (TAC), diacetyl cellulose, cellulose acetate butyrate, polyamide, polyimide, polyethersulfone, polysulfone, polypropylene, polymethylpentene, polyvinyl chloride, polyvinyl acetal, polyether ketone, polymethyl methacrylate, polycarbonate, polyurethane, and amorphous olefin (Cyclo-Olefin-Polymer (COP)). Among these, polyester is preferable from the aspect of easily improving mechanical strength. That is, the plastic film for optical use is preferably a polyester film.

[0164] Examples of the polyester constituting the polyester film include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polybutylene terephthalate (PBT). Among these, PET is preferable from the aspect of low intrinsic birefringence and easy reduction of in-plane retardation.

[0165] The plastic film may also contain additives such as an ultraviolet absorber, a light stabilizer, an antioxidant, an antistatic agent, a flame retardant, an anti-gelling agent, and a surfactant.

[0166] The thickness of the plastic film is preferably 15 μm to 60 μm, more preferably 20 μm to 55 μm, and further preferably 30 μm to 50 μm. By making the thickness 15 μm or more, the mechanical strength can be easily improved. In addition, by making the thickness 60 μm or less, the in-plane retardation can be easily reduced.

[0167] The haze of the plastic film for optical use according to JIS K7136:2000 is preferably 3.0% or less, more preferably 2.0% or less, and further preferably 1.0% or less.

[0168] In addition, the total light transmittance of the plastic film for optical use according to JIS K7361-1:1997 is preferably 80% or more, more preferably 85% or more, and further preferably 90% or more.

[0169] In order to improve the mechanical strength, the plastic film is preferably a stretched plastic film, more preferably a stretched polyester film. In addition, the stretched polyester film is more preferably a single-layer structure of a polyester resin layer.

[0170] The stretched plastic film can be obtained by stretching a resin layer containing the components constituting the plastic film. Examples of the stretching method include biaxial stretching such as sequential biaxial stretching and simultaneous biaxial stretching, and uniaxial stretching such as longitudinal uniaxial stretching. Among these, biaxial stretching, which can easily reduce the in-plane retardation and easily improve the mechanical strength, is preferred. That is, the stretched plastic film is preferably a biaxially stretched plastic film. In addition, among the biaxially stretched plastic films, a biaxially stretched polyester film is preferred, and a biaxially stretched polyethylene terephthalate film is more preferred. It should be noted that from the aspect of easily satisfying condition 2, the biaxially stretched plastic film preferably has stretching ratios in the flow direction and the width direction that are close to each other.

[0171] -Sequential biaxial stretching-

[0172] In sequential biaxial stretching, the cast film is stretched in the flow direction and then in the width direction of the film.

[0173] The stretching in the flow direction is usually carried out using the circumferential speed difference of the stretching rolls, and can be carried out in one stage or in multiple stages using multiple pairs of stretching rolls. From the aspect of suppressing excessive deviation of optical properties such as in-plane retardation, it is preferred to bring the plurality of pinch rolls close to the stretching rolls. The stretching ratio in the flow direction is usually 2 to 15 times, and from the aspect of suppressing excessive deviation of optical properties such as in-plane retardation, it is preferably 2 to 7 times, more preferably 3 to 5 times, and further preferably 3 to 4 times.

[0174] From the aspect of suppressing excessive deviation of optical properties such as in-plane phase difference, the stretching temperature is preferably the glass transition temperature of the resin to the glass transition temperature + 100°C. In the case of PET, it is preferably 70°C to 120°C, more preferably 80°C to 110°C, and further preferably 95°C to 110°C.

[0175] Regarding the stretching temperature, by rapidly raising the temperature of the film, etc., and shortening the stretching interval at low temperature, there is a tendency for the average value of the in-plane phase difference to become smaller. On the other hand, by slowly raising the temperature of the film, etc., and lengthening the stretching interval at low temperature, there is a tendency for the orientation to improve, the average value of the in-plane phase difference to increase, and the deviation of the slow axis to become smaller.

[0176] It should be noted that when heating during stretching, it is preferable to use a heater that generates turbulent flow. By heating with wind containing turbulent flow, a temperature difference is generated in a minute area within the film surface, and due to this temperature difference, a minute shift occurs in the orientation axis, and it becomes easy to satisfy Condition 3.

[0177] Functions such as slipperiness, adhesiveness, antistatic property, etc. can be imparted to the film stretched in the flow direction by on-line coating. In addition, before on-line coating, surface treatments such as corona treatment, flame treatment, plasma treatment, etc. can be carried out as needed.

[0178] The thickness of the coating film formed in this way during on-line coating is extremely thin, about 10 nm to 2000 nm (this coating film is stretched thinner by the stretching treatment). In this specification, such a thin layer is not counted as the number of layers constituting the plastic film.

[0179] Regarding the stretching in the width direction, generally, the tenter frame method is used, and while holding both ends of the film with jigs and conveying it, stretching is performed in the width direction. The stretching ratio in the width direction is usually 2 times to 15 times. From the aspect of suppressing excessive deviation of optical properties such as in-plane phase difference, it is preferably 2 times to 5 times, more preferably 3 times to 5 times, and further preferably 3 times to 4.5 times. In addition, compared with the longitudinal stretching ratio, it is more preferable to increase the width stretching ratio.

[0180] The stretching temperature is preferably the glass transition temperature of the resin to the glass transition temperature + 120°C, and the preferred temperature increases from the upstream to the downstream. Specifically, in the case of dividing the transverse stretching interval into two parts, the temperature difference between the upstream and the downstream is preferably 20°C or more, more preferably 30°C or more, further preferably 35°C or more, and even more preferably 40°C or more. In addition, in the case of PET, the stretching temperature in the first stage is preferably 80°C to 120°C, more preferably 90°C to 110°C, and further preferably 95°C to 105°C.

[0181] For the plastic film that has been subjected to stepwise biaxial stretching as described above, in order to impart flatness and dimensional stability, it is preferable to perform heat treatment within the range of the stretching temperature to below the melting point in a tenter. Specifically, in the case of PET, it is preferable to perform heat setting within the range of 150°C to 255°C, more preferably 200°C to 250°C. At this time, by performing heat setting at the highest possible temperature below the melting point, the crystallinity inside the film can be maintained. On the other hand, slightly reducing the crystallinity on the film surface can easily meet Condition 1. Additionally, from the aspect of suppressing excessive deviation of optical properties such as in-plane retardation, it is preferable to perform additional stretching of 1% to 10% in the first half stage of the heat treatment.

[0182] After heat-treating the plastic film, it is slowly cooled to room temperature and then wound up. Additionally, if necessary, relaxation treatment or the like can be combined during heat treatment or slow cooling. From the aspect of suppressing excessive deviation of optical properties such as in-plane retardation, the relaxation rate during heat treatment is preferably 0.5% to 5%, more preferably 0.5% to 3%, further preferably 0.8% to 2.5%, and even more preferably 1% to 2%. Additionally, from the aspect of suppressing excessive deviation of optical properties such as in-plane retardation, the relaxation rate during slow cooling is preferably 0.5% to 3%, more preferably 0.5% to 2%, further preferably 0.5% to 1.5%, and even more preferably 0.5% to 1.0%. From the aspect of flatness, the temperature during slow cooling is preferably 80°C to 150°C, more preferably 90°C to 130°C, further preferably 100°C to 130°C, and even more preferably 100°C to 120°C.

[0183] -Simultaneous biaxial stretching-

[0184] In simultaneous biaxial stretching, the cast film is introduced into a synchronous twin-screw tenter, and while holding both ends of the film with clamps and conveying it, stretching is performed synchronously and / or stepwise in the flow direction and the width direction. As synchronous biaxial stretching machines, there are the pantograph method, the screw method, the drive motor method, and the linear motor method, but the stretching ratio can be arbitrarily changed, and the drive motor method or the linear motor method that can perform relaxation treatment at any location is preferred.

[0185] The stretching ratio in simultaneous biaxial stretching is usually 6 to 50 times in terms of area ratio. From the aspect of suppressing excessive deviation of optical properties such as in-plane retardation, it is preferably 8 to 30 times, more preferably 9 to 25 times, further preferably 9 to 20 times, and even more preferably 10 to 15 times.

[0186] Additionally, in the case of simultaneous biaxial stretching, in order to suppress the in-plane orientation difference, it is preferable that the stretching ratios in the flow direction and the width direction are the same and the stretching speeds are also approximately equal.

[0187] From the aspect of suppressing excessive deviation of optical properties such as in-plane phase difference, the stretching temperature in synchronous biaxial stretching is preferably the glass transition temperature of the resin to the glass transition temperature + 120°C. In the case of PET, it is preferably 80°C to 160°C, more preferably 90°C to 150°C, and further preferably 100°C to 140°C.

[0188] For the film subjected to synchronous biaxial stretching, in order to impart planarity and dimensional stability, it is preferable to continue heat treatment at a temperature above the stretching temperature and less than the melting point in the heat setting chamber in the tenter. The conditions of this heat treatment are the same as those after sequential biaxial stretching.

[0189] <Flexural resistance>

[0190] The plastic film preferably does not develop cracks or fractures after 100,000 times of the folding test shown in the examples (more preferably after 300,000 times). In addition, for the plastic film, it is preferable that after 100,000 times of the folding test shown in the examples (more preferably after 300,000 times), when the measurement sample is placed on a horizontal table, the angle at which the end of the sample floats from the table is 20 degrees or less, more preferably 15 degrees or less. The angle at which the end of the sample floats of 15 degrees or less means that it is difficult to form creases due to folding. In addition, it is preferable that the plastic film shows the above results (no cracks, fractures, and creases caused by folding. The floating angle of the end of the sample after the test is 20 degrees or less.) in any direction of the flow direction and the width direction of the plastic film.

[0191] <Thickness>

[0192] From the aspect of mechanical strength, the plastic film for optics is preferably 10 μm or more, more preferably 20 μm or more, and further preferably 25 μm or more. In addition, from the aspect of reducing the in-plane phase difference, the plastic film for optics is preferably 100 μm or less, more preferably 75 μm or less, and further preferably 50 μm or less. It should be noted that from the aspect of improving the flexural resistance, it is also preferable that the thickness is 50 μm or less.

[0193] <Use>

[0194] As described above, the plastic film of the present invention can improve the scratch resistance when scratched with hard objects such as pencils and styli, and the scratch resistance when repeatedly rubbed with soft objects such as cloth. Therefore, the plastic film for optics of the present invention can be suitably used as the plastic film of an image display device, and particularly can be suitably used as the plastic film of an image display device equipped with a touch panel.

[0195] In addition, the plastic film of an embodiment of the present invention that satisfies Condition 2 or 3 can suppress the remaining of bending marks or the occurrence of breakage regardless of the bending direction, and thus can be suitably used as the plastic film of a curved image display device or a foldable image display device.

[0196] As the plastic film of the image display device, a plastic film that is used as the base material of various functional films such as a protective film for a polarization element, a surface protective film, an antireflection film, and a conductive film constituting a touch panel can be cited.

[0197] [Optical laminate]

[0198] The plastic film for optics of the present invention can also be made into an optical laminate by further forming functional layers such as a protective layer, an antireflection layer, a hard coat layer, an antiglare layer, a retardation layer, an adhesive layer, a transparent conductive layer, an antistatic layer, and an antifouling layer.

[0199] The functional layer of the optical laminate preferably includes an antireflection layer. The antireflection layer is preferably disposed on the outermost surface on the side of the plastic film having the functional layer.

[0200] By having an antireflection layer as the functional layer of the optical laminate, iridescence can be easily suppressed.

[0201] In addition, the functional layer more preferably includes a hard coat layer and an antireflection layer. When the functional layer includes a hard coat layer and an antireflection layer, the hard coat layer and the antireflection layer are preferably disposed in sequence on the plastic film for optics.

[0202] Common hard coat layers and antireflection layers can be applied to the hard coat layer and the antireflection layer.

[0203] [Polarizer]

[0204] The polarizer of the present invention has: a polarization element; a transparent protective plate A disposed on one side of the polarization element; and a transparent protective plate B disposed on the other side of the polarization element, wherein at least one of the transparent protective plate A and the transparent protective plate B is the plastic film for optics of the present invention.

[0205] The polarizer is used, for example, to impart antireflectivity by combination with a λ / 4 retardation plate. In this case, a λ / 4 retardation plate is disposed on the display element of the image display device, and the polarizer is disposed on the side closer to the observer than the λ / 4 retardation plate.

[0206] In addition, when the polarizer is used for a liquid crystal display device, it is used to impart the function of a liquid crystal light valve. In this case, the liquid crystal display device is configured in the order of a lower polarizer, a liquid crystal display element, and an upper polarizer, and the absorption axis of the polarization element of the lower polarizer and the absorption axis of the polarization element of the upper polarizer are arranged orthogonally. In this configuration, it is preferable to use the polarizer of the present invention as the upper polarizer.

[0207] <Transparent protective plate>

[0208] The polarizing plate of the present invention uses the above-mentioned plastic film for optical use of the present invention as at least one of the transparent protective plate A and the transparent protective plate B. In a preferred embodiment, both the transparent protective plate A and the transparent protective plate B are the above-mentioned plastic film for optical use of the present invention.

[0209] When one of the transparent protective plate A and the transparent protective plate B is the above-mentioned plastic film for optical use of the present invention, the other transparent protective plate is not particularly limited, and an optically isotropic transparent protective plate is preferred. Optically isotropic means that the in-plane retardation is 20 nm or less, preferably 10 nm or less, more preferably 5 nm or less. Examples of the transparent substrate having optical isotropy include an acrylic film and a triacetyl cellulose (TAC) film.

[0210] In addition, when one of the transparent protective plate A and the transparent protective plate B is the above-mentioned plastic film for optical use of the present invention, it is preferable to use the above-mentioned plastic film for optical use of the present invention as the transparent protective plate on the light-emitting side.

[0211] <Polarizing element>

[0212] Examples of the polarizing element include sheet-type polarizing elements such as a polyvinyl alcohol film, a polyvinyl formal film, a polyvinyl acetal film, and a saponified ethylene-vinyl acetate copolymer film that are dyed and stretched with iodine or the like; a wire grid-type polarizing element composed of a large number of parallel metal wires; a coating-type polarizing element coated with a lyotropic liquid crystal or a dichroic host-guest material; a multilayer thin film-type polarizing element, etc. It should be noted that these polarizing elements may be reflective polarizing elements having a function of reflecting and not transmitting polarized components.

[0213] The polarizing element is preferably arranged such that its absorption axis is substantially parallel or substantially perpendicular to the flow direction or the width direction of the plastic film for optical use. Substantially parallel means within ±5 degrees of 0 degree, preferably within ±3 degrees of 0 degree, more preferably within ±1 degree of 0 degree. Substantially perpendicular means within ±5 degrees of 90 degrees, preferably within ±3 degrees of 90 degrees, more preferably within ±1 degree of 90 degrees.

[0214] [Image display device]

[0215] The image display device of the present invention has: a display element; and a plastic film disposed on the light-emitting surface side of the above-mentioned display element, wherein the plastic film is the above-mentioned plastic film for optical use of the present invention.

[0216] Figure 4 And Figure 5 FIG. is a cross-sectional view showing an embodiment of the image display device 100 of the present invention.

[0217] Figure 4 and Figure 5 The image display device 100 has a plastic film 10 for optics on the light-emitting surface side of the display element 20 ( Figure 4 and Figure 5 on the upper side). Additionally, Figure 4 and Figure 5 The image display devices 100 of both have a polarization element 31 between the display element 20 and the plastic film 10 for optics. Additionally, Figure 4 and Figure 5 In, a transparent protective plate A (32) and a transparent protective plate B (33) are laminated on both sides of the polarization element 31. It should be noted that Figure 5 In the image display device of, the plastic film 10 for optics is used as the transparent protective plate A (32).

[0218] It should be noted that the image display device 100 is not limited to the Figure 4 and Figure 5 way. For example, Figure 4 and Figure 5 In, each component constituting the image display device 100 is arranged at a specific interval, but each component can also be integrated by an adhesive layer or the like. Additionally, the image display device may also have components not shown (other plastic films, functional layers, etc.).

[0219] <Display element>

[0220] As the display element, a liquid crystal display element, an EL display element (organic EL display element, inorganic EL display element), a plasma display element, etc. can be cited, and further, an LED display element such as a Micro LED display element can be cited.

[0221] When the display element of the display device is a liquid crystal display element, a backlight is required on the surface of the liquid crystal display element opposite to the resin sheet.

[0222] Additionally, the image display device can be an image display device having a touch panel function.

[0223] As the touch panel, methods such as a resistive film type, a capacitive type, an electromagnetic induction type, an infrared type, and an ultrasonic type can be cited.

[0224] The touch panel function can add a function within the display element like an in-cell touch panel liquid crystal display element, or a touch panel can be mounted on the display element.

[0225] In addition, as described above, the plastic film for optics of the present invention can suppress the residue of bending marks or the occurrence of breakage after the bending test. Therefore, when the image display device of the present invention is a curved image display device or a foldable image display device, it can exhibit more remarkable effects, and is thus preferred in this regard.

[0226] It should be noted that when the image display device is a curved image display device or a foldable image display device, the display element is preferably an organic EL display element.

[0227] <Plastic film>

[0228] The image display device of the present invention has the plastic film for optics of the present invention on the light-emitting surface side of the display element. This plastic film can be only one sheet, or two or more sheets.

[0229] As the plastic film disposed on the light-emitting surface side of the display element, there can be mentioned plastic films used as substrates for various functional films such as a polarization element protective film, a surface protective film, an antireflection film, and a conductive film constituting a touch panel.

[0230] <Other plastic films>

[0231] The image display device of the present invention can have other plastic films within the range that does not impair the effects of the present invention.

[0232] As the other plastic film, a plastic film having optical isotropy is preferred.

[0233] Examples

[0234] Next, the present invention will be described in more detail by way of examples, but the present invention is not limited by any of these examples.

[0235] 1. Measurement and evaluation

[0236] The atmosphere for the following measurement and evaluation is a temperature of 23°C ± 5°C and a humidity of 40% RH to 65% RH. In addition, before the measurement and evaluation, the sample is exposed in the above atmosphere for 30 minutes or more.

[0237] 1-1. In-plane retardation (Re), retardation in the thickness direction (Rth), and direction of the slow axis

[0238] Samples measuring 50 mm in the flow direction × 50 mm in the width direction were cut from the optical plastic films of the examples and comparative examples prepared or made from the following "2". For a total of 5 parts, namely 4 parts advancing 10 mm from the four corners of the cut sample towards the central part and the central part of the sample, the in-plane phase difference, the phase difference in the thickness direction, and the direction of the slow axis were measured. The averages of Re1 to Re5 calculated from the measurement results are shown in Table 1. The measuring device used was the product named "RETS-100 (measurement spot: diameter 5 mm)" manufactured by Otsuka Electronics Co., Ltd. Note that regarding the direction of the slow axis, the flow direction (MD direction) of the plastic film was taken as the reference 0 degrees, and the measurement was carried out in the range of 0 to 90 degrees.

[0239] 1-2. Indentation hardness of the cross-section

[0240] Two cut samples measuring 2 mm in the flow direction × 10 mm in the width direction were cut from the area of the sample measuring 50 mm in the flow direction × 50 mm in the width direction cut out from the above 1-1, and then, two such cut samples were Figure 1 embedded in resin as shown. Note that the embedded samples were made according to the preferred methods exemplified in (A1) and (A2) in the main text of the specification.

[0241] One embedded sample was cut perpendicularly to the flow direction with a diamond knife to produce Sample A for measuring the indentation hardness of the cross-section in the flow direction with the cross-section in the flow direction exposed. The other embedded sample was cut perpendicularly to the width direction with a diamond knife to produce Sample B for measuring the indentation hardness of the cross-section in the width direction with the cross-section in the width direction exposed. Note that the device for cutting the embedded samples used was the product named "Ultramicrotome EMUC7" manufactured by Leica Microsystems. In addition, the cut was made through the center of the cut sample in the embedded sample. Note that during cutting, it was first roughly cut (coarse trimming), and finally, it was precisely trimmed under the conditions of "speed: 1.00 mm / s" and "feed: 70 nm" so that the cross-section passing through the center of the sample was approximately flat. In addition, after precise trimming, a microscope was used to confirm that there were no foreign substances, unevenness, or other substances that would interfere with the measurement on the cross-section.

[0242] Next, using Sample A, the indentation hardness of the cross-section on the first surface side, the indentation hardness of the cross-section on the second surface side, and the indentation hardness of the cross-section at the exact middle in the thickness direction were measured, and MD1 and MD2 were calculated. Similarly, using Sample B, the indentation hardness of the cross-section on the first surface side, the indentation hardness of the cross-section on the second surface side, and the indentation hardness of the cross-section at the exact middle in the thickness direction were measured, and TD1 and TD2 were calculated. The results are shown in Table 1. As described in the main text of the specification, MD1, MD2, TD1, and TD2 refer to the average values of 5 measurement values.

[0243] As Figure 2 shown, the indentation hardness of the cross-section on the first surface side and the indentation hardness of the cross-section on the second surface side are measured at positions 2.0 μm inside from the first surface and the second surface.

[0244] It should be noted that regarding the indentation hardness, a Berckovich indenter (material: diamond triangular pyramid) is vertically pressed into the cross-section, and a product numbered "TI950 TriboIndenter" manufactured by HYSITRON is used as the measuring device, and the measurement is carried out under the following conditions using the application software (TriboScan Version 9.6.0.2) attached to this device.

[0245] In addition, in the above measurement, the following standard adjustment is implemented: Before measuring the indentation hardness of each sample, an indentation test is carried out using a standard specimen (fused silica (5 - 0098) manufactured by HYSITRON) with known indentation hardness and complex elastic modulus, and it is confirmed that the indentation hardness and complex elastic modulus obtained from the test results are within the reference values. That is, after implementing the above standard adjustment, for sample A, the indentation hardness of the cross-section on the first surface side, the indentation hardness of the cross-section on the second surface side, and the indentation hardness of the cross-section at the exact middle in the thickness direction are measured 5 times respectively. And after completing the measurement of sample A, the above standard adjustment is implemented again, and for sample B, the indentation hardness of the cross-section on the first surface side, the indentation hardness of the cross-section on the second surface side, and the indentation hardness of the cross-section at the exact middle in the thickness direction are measured 5 times respectively.

[0246] It should be noted that in Table 1, the case where condition 1 (both MD2 / MD1 and TD2 / TD1 exceed 1.01 and are 1.30 or less) is satisfied is denoted as "Y", and the case where condition 1 is not satisfied is denoted as "N".

[0247] <Measurement Conditions>

[0248] · Indenter used: Berckovich indenter (model: TI - 0039, manufactured by HYSITRON)

[0249] · Indentation condition: Displacement control mode

[0250] · Maximum indentation depth: 200 nm

[0251] · Load application time: 20 seconds (speed: 10 nm / second)

[0252] · Holding time: Hold at the maximum indentation depth for 5 seconds

[0253] · Load unloading time: 20 seconds (speed: 10 nm / second)

[0254] 1 - 3. Abrasion resistance 1 (Abrasion resistance against hard objects)

[0255] Press the test pencil with hardness F specified in JIS S6006 onto the surface of the optical plastic films of the examples and comparative examples, and conduct the pencil hardness test (4.9 N load) specified in JIS K5600 - 5 - 4:1999. The test is carried out in two directions: the flow direction and the width direction. As a result, when there are no scratches on the surface of the plastic film in all directions, it is recorded as "A", and when scratches are generated on the surface of the plastic film in at least one direction, it is recorded as "C".

[0256] 1 - 4. Abrasion resistance 2 (Abrasion resistance against soft objects)

[0257] Press the flannel cloth of cotton No. 300 onto the surface of the optical plastic films of the examples and comparative examples, and after reciprocating friction 1000 times with a load of 500 g / cm 2 Visually confirm the presence or absence of scratches under the illumination of a fluorescent lamp. The test is carried out in two directions: the flow direction and the width direction. The test device used is a Gakushin abrasion tester (product number "AB - 301" manufactured by TESTER SANGYO Co., Ltd.). As a result, when there are no scratches on the surface of the plastic film in all directions, it is recorded as "A", and when scratches are generated on the surface of the plastic film in at least one direction, it is recorded as "C".

[0258] 1 - 5. Flexural resistance

[0259] <Width direction>

[0260] Cut out strip - shaped samples with a short side (width direction) of 30 mm × long side (flow direction) of 100 mm from the optical plastic films of the examples and comparative examples. Fix both ends of the short - side (30 mm) side of the sample to a durability tester (product name "DLDMLH - FS", manufactured by YUASA SYSTEM Co., Ltd.) (fix the area 10 mm from the front end), and conduct a continuous folding test of folding 180 degrees 100,000 times. The folding speed is set to 120 times per minute. A more detailed method of the folding test is shown below.

[0261] After the folding test, place the strip - shaped sample on a horizontal table, and measure the angle at which the end of the sample floats from the table. The results are shown in Table 1. It should be noted that when the sample breaks during the process, it is recorded as "broken".

[0262] <Flow direction>

[0263] Cut out strip - shaped samples with a short side (flow direction) of 30 mm × long side (width direction) of 100 mm from the optical plastic films of the examples and comparative examples, and conduct the same evaluation as above.

[0264] <Details of the folding test>

[0265] As shown in Figure 6 (A), in the continuous folding test, first, the side portions 10C of the plastic film 10 and the side portions 10D facing the side portions 10C are respectively fixed by the fixing portions 60 arranged in parallel. The fixing portions 60 can slide and move in the horizontal direction.

[0266] Next, as shown in Figure 6 (B), the fixing portions 60 are moved closer to each other, thereby deforming and folding the plastic film 10. Then, as shown in Figure 6 (C), the fixing portions 60 are moved to a position where the distance between the two opposing side portions fixed by the fixing portions 60 of the plastic film 10 is 2 mm. After that, the fixing portions 60 are moved in the reverse direction to eliminate the deformation of the plastic film 10.

[0267] By moving the fixing portions 60 as shown in Figure 6 (A) to (C), the plastic film 10 can be folded 180 degrees. In addition, the continuous folding test is performed in such a manner that the bent portion 10E of the plastic film 10 does not protrude from the lower end of the fixing portion 60, and the distance when the fixing portions 60 are closest to each other is controlled to 2 mm, whereby the distance between the two opposing side portions of the optical film 10 can be made 2 mm.

[0268] 1 - 6. Iris spots

[0269] Samples cut from the plastics for optics of the examples and comparative examples (the samples produced in 1 - 1) are arranged on the viewing - side polarizer of the image display device configured as follows, and the TD direction of the sample is made parallel to the horizontal direction of the screen. Then, the image display device is lit in a dark - room environment, and it is observed with the naked eye from various angles, and the presence or absence of iris spots is evaluated according to the following criteria.

[0270] A: No iris spots can be observed.

[0271] B: Iris spots are observed in a very small part of the region.

[0272] C: Iris spots are observed in most of the region.

[0273] <Configuration of the image display device>

[0274] (1) Backlight: White LED or cold cathode tube

[0275] (2) Light - source - side polarizer: Having a TAC film as the protective films on both sides of the polarizing element composed of PVA and iodine. It is arranged such that the direction of the absorption axis of the polarizing element is perpendicular to the horizontal direction of the screen.

[0276] (3) Image display unit: Liquid crystal unit

[0277] (4) Observation side polarizer: A polarizer using a TAC film as a protective film for a polarizing element composed of PVA and iodine. It is configured such that the direction of the absorption axis of the polarizing element is perpendicular to the parallel direction of the screen.

[0278] (5) Dimensions: 10 inches diagonal

[0279] 1 - 7. Blackening

[0280] Samples cut from optical plastics of the examples and comparative examples (samples made in 1 - 1) are arranged on the observation side polarizer of the image display device having the configuration shown in 1 - 6, such that the TD direction of the samples is parallel to the horizontal direction of the screen. Then, with the image display on which the samples are arranged in the vertical direction, the image display devices made in the examples and comparative examples are observed from the front through a polarizing sunglasses that absorbs S - polarized light, and the blackening is evaluated according to the following criteria.

[0281] A: No blackening in all regions.

[0282] B: Blackening occurs in a very small part of the region.

[0283] C: Blackening occurs in most of the region.

[0284] 2. Production and preparation of stretched polyester film

[0285] [Example 1]

[0286] 1 kg of PET (melting point 258 °C, absorption center wavelength: 320 nm) and 0.1 kg of an ultraviolet absorber (2,2'-(1,4 - phenylene)bis(4H - 3,1 - benzoxazin - 4 - one)) are melt - mixed at 280 °C using a kneader to produce pellets containing the ultraviolet absorber. The pellets and PET with a melting point of 258 °C are put into a single - screw extruder, melt - kneaded at 280 °C, extruded from a T - die, and cast onto a casting drum with a surface temperature controlled at 25 °C to obtain a cast film. The amount of the ultraviolet absorber in the cast film is 1 part by mass relative to 100 parts by mass of PET.

[0287] The obtained cast film is heated by a roller set set at 95 °C, and then, while heating the film from both sides using a radiation heater such that the film temperature at the 150 - mm position of a 400 - mm stretching section (starting from stretching roller A and ending at stretching roller B. Stretching rollers A and B each have 2 nip rollers) reaches 103 °C, the film is stretched 3.3 times in the flow direction and then temporarily cooled. It should be noted that when heating with the radiation heater, air at 92 °C and 4 m / s is blown onto the film from the side opposite to the radiation heater, thereby generating turbulence on the front and back of the film and disturbing the temperature uniformity of the film.

[0288] Next, corona discharge treatment is performed on both sides of the uniaxially stretched film in air. The wetting tension of the base film is set to 55 mN / m. For the corona discharge treatment surfaces on both sides of the film, a slip layer coating liquid "containing a polyester resin with a glass transition temperature of 18°C, a polyester resin with a glass transition temperature of 82°C, and silica particles with an average particle diameter of 100 nm" is coated online to form a slip layer.

[0289] Next, the uniaxially stretched film is introduced into a tenter frame, preheated with hot air at 95°C, and then stretched 4.5 times in the film width direction at a temperature of 105°C in the first stage and 140°C in the second stage. Here, when the transverse stretching section is divided into two parts, stretching is performed in two stages in such a way that the stretching amount of the film at the midpoint of the transverse stretching section (the film width at the measurement location - the film width before stretching) is 80% of the stretching amount at the end of the transverse stretching section. For the transversely stretched film, heat treatment is directly performed in the tenter frame in stages using hot air at a heat treatment temperature of 245°C starting from 180°C. Then, a 1% relaxation treatment is performed in the width direction under the same temperature conditions, and then it is rapidly cooled to 100°C, and then a 1% relaxation treatment is performed in the width direction. After that, it is wound up to obtain the optical plastic film (biaxially stretched polyester film, thickness 40 μm) of Example 1.

[0290] [Example 2]

[0291] The location where the film temperature reaches 103°C is changed to the 200 - mm location of the 400 - mm stretching section. Otherwise, the optical plastic film (biaxially stretched polyester film, thickness 40 μm) of Example 2 is obtained in the same manner as in Example 1.

[0292] [Example 3]

[0293] The thickness of the cast film of Example 1 is increased, the stretching ratio in the flow direction is changed from 3.3 times to 3.5 times, and the stretching ratio in the width direction is changed from 4.5 times to 5.0 times. Otherwise, the optical plastic film (biaxially stretched polyester film, thickness 50 μm) of Example 3 is obtained in the same manner as in Example 1.

[0294] [Example 4]

[0295] The thickness of the cast film of Example 1 is increased, the stretching ratio in the flow direction is changed from 3.3 times to 3.5 times, and the stretching ratio in the width direction is changed from 4.5 times to 5.0 times. Otherwise, the optical plastic film (biaxially stretched polyester film, thickness 42 μm) of Example 4 is obtained in the same manner as in Example 1.

[0296] [Comparative Example 1]

[0297] As the plastic film for optics of Comparative Example 1, a commercially available biaxially stretched polyester film (manufactured by Toyobo Co., Ltd., trade name: Cosmoshine A4100, thickness: 50 μm) was prepared.

[0298] [Comparative Example 2]

[0299] As the plastic film for optics of Comparative Example 2, a biaxially stretched polyester film having a three-layer structure (a three-layer of crystalline polyester / amorphous polyester / crystalline polyester) of Example 13 of Japanese Unexamined Patent Application Publication No. 2018-59078 was produced.

[0300] [Table 1]

[0301] Table 1

[0302]

[0303] From the results in Table 1, it was confirmed that the plastic films for optics of Examples 1 to 4 that satisfied Condition 1 could improve the scratch resistance for both hard and soft objects. In addition, the plastic films for optics of Examples 1 to 4 could suppress blackening by satisfying Condition 3, and furthermore, regardless of the bending direction, the residual bending marks or breakage after the bending test could be suppressed. In addition, it was confirmed that the plastic films for optics of Examples 1 and 2 could suppress iridescence by satisfying Condition 4.

Claims

1. A plastic film for optics, which has a first surface and a second surface located on the side opposite to the first surface, and satisfies the following Condition 1, <Condition 1> Regarding the indentation hardness of the cross-section in the flow direction of the plastic film, the softer hardness among the indentation hardness of the cross-section on the first surface side and the indentation hardness of the cross-section on the second surface side is defined as MD1, and the indentation hardness of the cross-section at the exact middle in the thickness direction is defined as MD2; in addition, regarding the indentation hardness of the cross-section in the width direction of the plastic film, the softer hardness among the indentation hardness of the cross-section on the first surface side and the indentation hardness of the cross-section on the second surface side is defined as TD1, and the indentation hardness of the cross-section at the exact middle in the thickness direction is defined as TD2; on this premise, both MD2 / MD1 and TD2 / TD1 exceed 1.01 and are 1.30 or less.

2. The plastic film for optics according to claim 1, which further satisfies the following Condition 2, <Condition 2> When the larger one of the product of MD1 and MD2 and the product of TD1 and TD2 is defined as X1 and the smaller one is defined as X2, X1 / X2 is 1.30 or less.

3. The plastic film for optics according to claim 1 or 2, which further satisfies the following Condition 3, <Condition 3> Cut out a sample of 50 mm in the flow direction × 50 mm in the width direction from the plastic film; measure the directions of the slow axes at a total of 5 positions including 4 positions advancing 10 mm from the four corners of the sample toward the central part and the central part of the sample; when the angles formed by either the flow direction or the width direction of the sample and the directions of the slow axes at each measurement position are respectively defined as D1, D2, D3, D4, D5, the difference between the maximum value and the minimum value of D1 to D5 is 5.0 degrees or more.

4. The plastic film for optics according to any one of claims 1 to 3, which further satisfies the following Condition 4, <Condition 4> Cut out a sample of 50 mm in the flow direction × 50 mm in the width direction from the plastic film; measure the in-plane phase differences at a total of 5 positions including 4 positions advancing 10 mm from the four corners of the sample toward the central part and the central part of the sample; when the in-plane phase differences at the 5 positions are respectively defined as Re1, Re2, Re3, Re4, Re5, the average of Re1 to Re5 is 500 nm or less.

5. The plastic film for optics according to any one of claims 1 to 4, which further satisfies the following Condition 5, <Condition 5> Cut out a sample of 50 mm in the flow direction × 50 mm in the width direction from the plastic film; measure the phase differences in the thickness direction at a total of 5 positions including 4 positions advancing 10 mm from the four corners of the sample toward the central part and the central part of the sample; when the phase differences in the thickness direction at the 5 positions are respectively defined as Rth1, Rth2, Rth3, Rth4, Rth5, the average of Rth1 to Rth5 is 2000 nm or more.

6. A polarizing plate, which has: a polarizing element; a transparent protective plate A disposed on one side of the polarizing element; and a transparent protective plate B disposed on the other side of the polarizing element, wherein, At least one of the transparent protection plate A and the transparent protection plate B is the plastic film for optics described in any one of claims 1 to 5.

7. An image display device, comprising: a display element; and a plastic film disposed on the light-emitting surface side of the display element, wherein, the plastic film is the plastic film for optics described in any one of claims 1 to 5.

8. The image display device according to claim 7, further comprising a polarization element between the display element and the plastic film.

Images